Thursday, December 31, 2009

LiveFolders exist in Android since version 1.5. The mechanism was advertised as a lead feature of Android 1.5/1.6 but somehow it failed to take hold. There are excellent introductions to LiveFolders, for example this one so I cut short here. Folder is an element of the Android desktop (technically, the Launcher application) that contains "links" to e.g. Android applications. More exactly, these "links" fire Intents and those intents can launch applications, open data sources, etc. A LiveFolder is different from a normal folder in that the content of the folder (folder items and their graphical representations) is not statically stored but is generated dynamically by a ContentProvider. There is a discovery mechanism based on Activities handling the android.intent.action.CREATE_LIVE_FOLDER intent. The Launcher discovers all the Activites handling this intent, invokes them by sending the intent, in response these Activities return the URIs of the LiveFolders' ContentProviders. If the user creates an icon for a LiveFolder and opens this icon, the Launcher will know the URI of the ContentProvider behind the LiveFolder, will list its content (as icon grid or list) and allow the user to activate an item in the LiveFolder which will fire an intent, opening a contact for example.

So far so good but I would like you to think a bit further. So we have these LiveFolders that

Allow applications to produce items of interest

And these items of interest, when "clicked", activate other actions.

Doesn't is sound familiar? Feeds in web programming are just like that. Android UI already has some elements of web programming (like the Back button) and now it has feeds. Feeds are not only fashionable nowadays but have deep roots in dataflow programming (here is a short introduction if you want to know more) and clever application frameworks like Yahoo Pipes were built around them.

Sadly, exploiting this possibility is not easy now. The only LiveFolder client I am aware of is Launcher itself and aggregating/manipulating feeds is not easy. I am attempting to do just that in this blog post. I will show you how to create a LiveFolder of non-starred contacts of two existing LiveFolders: All contacts and starred contacts.

Did you know that you can star contacts? I did not but there is a LiveFolder for starred contacts.

Install the application. Now if you select the Add menu on the Launcher main screen, select Folders, you will see the Nonstarred folder. Add that folder and it appears on the home screen. If you open that folder, you will see all non-starred contacts in it. If you click any such contact, the Contact application is opened and the contact is displayed.

We generate the non-starred LiveFolder from to other LiveFolders. The application has two parts, an Activity that is invoked by Launcher to discover the LiveFolder. This is the part of the application that itself discovers other LiveFolders to retrieve the URIs of the two source LiveFolders. These URIs are then passed to the ContentProvider that aggregates the two source LiveFolders into a third one. The interesting part is the fake Cursor we return to the LiveFolder client which really accesses in-memory data and not SQLite database as it is usual with Android applications.

As a bonus track, I share with you a simple LiveFolder client that helps you to discover other LiveFolders.

So far so good but I am not satisfied. In order to achieve the ease of manipulating RSS feeds in web programming, more automation is necessary. I intend to do it later, after I experimented with another idea.

About the blog

This blog is a personal diary about my adventures with the Google Android platform. I write it in the hope that others may find my experiences useful but please, beware. The blog is created as I gain experience about the platform myself so errors, omissions, etc. may be found in the entries.